Fabry-Perot etalons have been used for sensing and measuring strain. In one method a low-coherence light source is coupled into an optical fiber that contains a Fabry-Perot etalon. The Fabry-Perot etalon is attached to the device undergoing strain. The output of the Fabry-Perot etalon is fed into a Michelson interferometer. Michelson interferometers measure coherence and have the characteristic that when the two optical paths of the Michelson interferometer are equal in length a fringe (interference) pattern is formed at the output of the Michelson interferometer. A second fringe pattern occurs when the difference between the two internal optical path lengths is equal to the optical path length of the Fabry-Perot etalon contained in the fiber optic cable. This occurs because the optical path length between one of the etalon mirrors and one of the Michelson interferometer's mirrors is equal to the optical path length between the other etalon mirror and the other Michelson interferometer's mirror. When the optical path length of the Fabry-Perot etalon changes due to the strain of the test device, this shows up as shift in the fringe (interference) pattern. The only way to measure this difference is to calibrate the gears, driving the mirrors, that vary the optical path length inside the Michelson interferometer. Unfortunately, this results in only low resolution measurements of the optical path lengths and limits the system to low resolution strain measurements.
Previous, relative, high resolution strain measurement systems have been designed by replacing the low-coherence light source with a high-coherence light source. The high coherence light source produces a fringe pattern over a relatively large range of optical path length differences. When the optical path length of the Fabry-Perot etalon is changed, the fringes or variations in lines of darkness and light move across the image plane of the Michelson interferometer. The movement of one fringe (light line to light line) corresponds to one wavelength of change in the optical path length. Thus it is possible, by counting or measuring the movement of the fringe patterns, to accurately and with high resolution, determine the change in optical path length and therefore the strain of the device under test. Unfortunately, the fringe pattern looks essentially the same over large differences in optical path lengths. As a result, it is impossible to measure the optical path length of the Fabry-Perot etalon before attaching it to the device under test. While it is possible to count fringes as the Fabry-Perot etalon is attached to the device under test, this may not be possible for a variety of reasons. For instance, the device may be undergoing such rapid change in strain that the fringes cannot be counted, or it is necessary to measure the strain of the device over long periods of time, which requires not losing count of the fringes. This may not be practical due to short term perturbations in the device under test or loss of power.
Thus there exists a need for a high resolution, accurate strain measurement system that can also determine the absolute strain.